The present invention generally relates to vascular grafts that have outside surfaces which have porous structures, the vascular grafts being of the non-braided and non-woven type and having an outside surface porosity that provides an environment which is conducive to tissue ingrowth into the pores of the porous surface. The vascular grafts are formed from a fiber forming polymer which is extruded into fibers that are wound or spun onto a mandrel while being intermittently subjected to electrostatic charge conditions. Typically, breaks occur in the formed fiber during spinning, and the broken fiber is readily reattached at or near its point of breakage with the aid of the electrostatic conditions. Such electrostatic conditions help to provide vascular grafts with good interfiber bonding and closely controlled pore sizes, especially by providing internal surfaces that are exceptionally smooth and that have low porosity or substantially no porosity, as desired.
Vascular grafts are known to be made by methods which include winding extruded material onto a mandrel in an attempt to provide a degree of porosity that is desired for implantable vascular grafts, especially including providing an ingrowth environment that is particularly suitable for promoting tissue ingrowth at the implantation locations. One such approach is detailed in U.S. Pat. No. 4,475,972, the disclosure of which is incorporated by reference hereinto. That patent describes non-woven vascular grafts that are made by extruding a polyurethane solution under pressure through an opening and then drawing the extruded material while winding same on a mandrel.
In practicing methods such as those of this patent, certain problems have been encountered. It is not always possible to adequately control the extruded fibers in order to obtain both small pore size and good adhesion between connecting spun fibers. It is particularly difficult to achieve adequate bonding of the first few graft layers, which contributes to subsequent tearing thereof during mandrel removal, both of which result in the production of vascular grafts that have internal surfaces which are uneven and scraggly with torn fibers extending into the lumen of the graft. It is typically very desirable that vascular grafts have an internal surface that is substantially smooth and as free of obstructions as possible in order to thereby provide flow-through conditions that are substantially optimum for the internal diameter of the vascular graft.
When making these types of wound spun fiber vascular grafts, the fibers often break during the spinning operation, especially when, as is usually preferred, the spun fibers are drawn longitudinally during spinning, which exerts a force on the freshly extruded and as yet only partially set filament in a manner that tends to occasionally break the filament. When such fiber breakage occurs under these circumstances or any other circumstances (such as inertial effects, bubbles in polymer or the like), it is typically necessary to stop the relative movement between the extrusion device and the mandrel and to manually reattach the broken fiber end to the remainder of the fiber(s) on the mandrel or to the mandrel itself.
Besides being labor intensive and time-consuming and potentially leading to production down-time, this need for manual reattachment often leads to poor fiber adhesion at the plane of reattachment, which can lead to graft delamination. Fiber breaks that are reattached according to previously known methods include reattaching the fiber at the ends of the graft, which ends are later cut off and discarded. This procedure to manually reattach a fiber without breaking other fibers is of such a long duration that the underlying fiber layer dries out and does not bond well to subsequent layers. Uniformity of spun strands can thus be disrupted, leading to potential points of weakness or unsightly sections of the completed vascular graft. This is especially important for vascular tubes that are intended for arterial use inasmuch as arterial prostheses should be able to withstand pulsatile arterial blood pressures of at least 300 mm Hg, preferably greater than 500 mm Hg, for prolonged periods of time of on the order of ten years and more.
Included in the objectives of providing vascular grafts having porous surfaces is to promote, after implantation of such a graft, colonization and tissue ingrowth into the depth of the porous surface from adjacent body tissue in order to provide bonding between the body tissue host and the porous vascular graft. Typically, the body tissue ingrowth is combined with the promotion of tissue growth into the porous surface from nucleated bloodstream cells. Such porous surfaces provide a porous depth that affords a means for fixation to host tissues by soft tissue ingrowth into the porous depth of the surface, and they provide tissue-implant interfaces which are blood compatible arising from colonization and tissue formation on the blood-contacting surfaces of the vascular grafts. Tissue ingrowth is desired on the external surface. Desired on the internal surface is endothelium cell ingrowth which is provided by nucleated blood cells. Accordingly, it is important that such vascular grafts be provided with porosity and that the parameters of such porosity be closely controlled during manufacture of the vascular grafts in order to achieve the desired extent of ingrowth after implantation.
Disadvantages of the type hereinbefore discussed have been substantially eliminated by proceeding in accordance with the present invention, which also provides vascular grafts that are formed under closely controlled conditions whereby the porosity can be varied from layer to layer while also being accurately and consistently formed. For example, the present invention achieves fiber break reattachment while minimizing the chance of forming undesirable delamination sites. The invention also facilitates the formation of a vascular graft having a substantially smooth inner surface that is of low porosity or that is substantially non-porous. Also, an electrostatic field is used to provide a well-bonded layer on the outside of the graft to prevent scraggylness of the graft from handling during implantation. A single pass on the outside of the graft provides a "hair net" effect without affecting the pore size.
In summary, the present invention includes vascular grafts that are made by extruding a fiber forming polymer while winding same on a mandrel and while intermittently applying an electrostatic charge between the extrusion device and the mandrel in order to accelerate the movement of the fiber forming polymer strand toward the mandrel. Typically, the electrostatic charge is applied during extrusion and winding of the first or innermost spinning passes in order to provide a vascular graft having a substantially smooth interior surface and low porosity inner layer or layers. Electrostatic charge application at the times of fiber breakage during any portion of the spinning operation accelerates the broken fiber from the extruder and onto the mandrel at or near the location of breakage and before the broken ends have set to such a degree that smooth reattachment is no longer possible.
It is accordingly a general object of the present invention to provide an improved non-woven vascular graft.
Another object of the present invention is to provide an improved vascular graft having an exterior surface that promotes tissue ingrowth thereinto and having an internal surface of reduced porosity that allows cellular ingrowth when compared with the external surface.
Another object of this invention is to provide an improved vascular graft that includes the extrusion and winding of a fiber forming polymer onto a mandrel while applying an electrostatic charge in order to accelerate the forming fiber onto the mandrel at selected times.
Another object of the present invention is to provide an improved vascular graft and the production thereof by a process including the intermittent application of electrostatic energy between a mandrel and an extruder for a fiber forming polymer.
Another object of the present invention is to provide an improved process for the production of vascular grafts which includes closely controlling the porosity of various layers of the grafts.
Another object of the present invention is to provide an improved process for manufacturing non-woven vascular grafts and which includes electrostatically repairing breaks in fiber forming polymer extrusions.
These and other objects, features and advantages of this invention will be clearly understood through a consideration of the following detailed description.